Process of bending wood



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2,874,753 PROCESS OF BENDING WOOD William L. Knowles, Murfreeshoro, N. C. No Drawing. Application April 24, 1956 Serial N0. 580,171 2 Claims. (Cl. 154-33.05)

. This invention relates to a method or process of bending wood. More particularly, the invention relates to what I call dry bending as distinguished from the heretofore known methods and processes wherein wood is steamed, soaked, steeped or otherwise similarly treated to introduce free moisture into the wood before bending the wood.

The known procedures present many hazards and complications, as for instance, the wood must be retained in the shape bent until nominal dryness is attained, requiring forms or molds and much time. Even after being held into bent shape until nominal dryness is attained, the bent shape has a tendency to revert to a flat state after being released from its retainers, particularly when subjected to varying degrees of humidity or to moisture by the application of finishes or other manufacturing processes. Shrinkage, especially tangentially or across the grain is encountered in the evaporating or drying excessive free moisture from the wood and these shrinkage forces are so great as to split the wood fibres and distort the product if not render it useless for'the intended purpose. These complications are apparent in simple arcs where the grain of the wood is parallel to the axis of the curve. The same problems multiply when the wood'is formed into U-shapes, reverse curves or a series of them, compound curves. Even more problems are encountered when the bend is perpendicular to the grain of the wood.

The primary object in bending wood is for it to retainits shape for some useful purpose. It is common belief that thin wood bends more readily and to shorter radii than thicker members. All known authorities on wood technology cite that wood cannot successfully be bent on a radius. of less than 20 to 25 times its own thickness. Hence, to achieve many useful and artistic shapes, particularly short curvatures, very thin wood is used, so thin as to be useless of itself for the purpose. intended. In order to. stabilize, support or otherwise make useful the shape, the wood is contemplated in more than one layer, or in' combination with other materials that will lend stability and strength, such as: paper, plastic, wood, metals, cloth, glass, etc. This contemplation introduces the technique of adhering the material together such as by gluing, which in turn presents additional complications depending upon the use, stability, quality, appearance, etc., to be achived. There must be uniform and continuous pressure over the entire area to be bondedhlding consistently until the bond is achieved. While many adhesives may be employed which can be accelerated in drying or curing by the application of heat, such heat beyond 212 F. will create steam from all the moisture thus impairing bonds. There are many adhesives that will onlycure under heat, but with the moisture present to effect the bend,- heat introduces steam as well as rapid' tangerial shrinkage. Dry .film glues may be used, such are of a thermosetting nature and it is a well known fact that moisture must be very low to use this type with success. Again strictly aside from any mechanical means employed to achieve a 'given' shape 'in wood, we must determine the composition of wood in order to arrive at a process that overcomesthe problems foregoing outlined.

First, exhaustive studies have been made on most species of known wood as to their location, growth, size, and availabilityjqualities and characteristics, such as strength,

texture, weight-green and dry; tendencies toward shrink--- 2,874,753 ,P etedfeb- 2 1,1

finishing, boring, cutting, chipping; and other analysis for workability such as gluing characteristics. Most wood is a combination by nature in the main of cellulose and lignin. Other components are not important for purposes of this analysis and process. It contains two types of moisturein my description, it is called free moisture and cell moisture.

It is a well known process to dry wood after it has been rendered from a tree. One method is to air dry and the other (usually following the first) to mechanically dry,

such as in a kiln. Both of these perform the function ofreducing the free moisture content of the wood. It is performed for the purpose of stabilizing the'wood by reducing its tendencies to warp, twist and shrink. Mechanical drying may be carried to the extent of rendering the wood oven dry, in other words, having no free moisture. It is easier to add moisture to undried wood than it is to wood that has once been dried. However, even if wood is oven dry it will relatively quickly reassume the relative moisture content dictated by the atmospheric conditions where it is placed. In the process of mechanical drying, it is a well known art that the applications of the high temperature, too quickly, will case harden the wood. This is similar to the surface hardening of metals. The reasons for this Will be apparent later herein.

Of the cell moisture, over 90% of the cellitself is drying, the high sudden heat has dried out the free mois ture from the surface fibers sufliciently quick to accomplish a flow and set of the cells in the surface before free moisture in the inner areas have elevated and temperature sufiicient to drive off the free moisture in the inner areas.

To bend wood properly and keep it bent, I have found as follows:

(1) Dry the wood and keep it as dry as practicable prior to bending.

(2) Pre-heat-to obtain as much penetration as possible-without risking the time or temperature required for case hardening-just prior to bending.

(3) In the bending process the heat must be in direct contact with the wood for the'quickest penetration and must beof sulficient temperatures to penetrate the core before set on the surface.

To illustrate:

A $5 inch sheet of wood at 10% moisture content when exposed to a free dry air circulating at 240 P. will reduce to oven dry in approximately 5 seconds. During or immediately thereafter, a pressure contact heat at 290 F. is applied. The flow of the lignin throughout the structure will of itself soften the fibers of the wood and permit the forming without rupture. Once held in close contact with this heat, and for approximately 5 seconds, the material will stay to the shape formed.

A inch sheet of wood under the above conditions will require 10 seconds of preheat at 250 F. and approxi-' In other words, the thicker the Wood, the more vari-' ance in time and temperature. This may be. carried on until a piece of wood 2 inches square would have to be subjected to such high temperatures, to obtain flow at the core, as to scorch or char the surface. When such char is removed, however, it will be found that the wood is hard and dense because of the flow and set of the lignin.

Thus there are practicable limits depending upon the use of the product.

The applications of great pressure on wood will crush the fibre. This pressure along with high heat in the absence of excessive moisture will rupture the cells and cause a flow of the cell contents throughout the surface fibre, thus causing a slick, hard, relatively non-porous surface. There are instances where resins are added to the wood either by impregnation or on the surface and the same high pressure and temperature creating a product, no longer having the appearance or the characteristics of wood.

These same processes may be adopted to bend wood. To a degree, the same objective is achieved bybending. Even with only sufiicient pressure of the bending elements on the Wood to create its shape, there are terrific pressures brought on some of the Wood cells. On one side of every bend, certain cells are under compression and the other under tension. The degree of the bend relative to the thickness of the material determines the extent of these pressures. With relatively short bends the cell rupture is sufficient to cause cell flow to an extent that the characteristics of the Wood have been altered. This penetration from flow assists in shaping the material and creating stability in its new form.

The obstacles overcome and the objectives achieved by this process eliminates all the hazards cited under the known process and accepted practices of bending" when green or wet.

(1) The shape or bend is achieved quickly.

(2) The shape will stay as the chemical and mechanical structure has been altered.

(3) There is no noticeable shrinkage in the process, or after, of bending.

(4) Since full shapes are made at all times, there is no deformation to impair multiple bends, compound, curves, etc.

(5) Dryness and heat is conducive of good bending quickly-hence strength and stability may be achieved quickly in rare shapes, and with arcs on a radius as low as 4 times thickness of the Wood.

Single sheets of wood thus bent may have many practical applications and uses, and while bends thus made will retain their shape it is well known that the thinner the wood the easier to break or split particularly with the grain.

To employ this process of bending and at the same time make material of greater stability and usefulness, we employ the art of laminating. This may be other layers of wood, paper, plastic, metal, cloth, etc. However, as cited above, problems are encountered because generally speaking moisture is added by the use of most of the practicable adhesives required to achieve the bond between the lamina.

To overcome this disadvantage from added moisture, the least possible amount of adhesive is employed consistent with the types of material to be laminated. -Also the adhesives should be of as high a solid content as practicable and a catalyst or hardener used to complement the preheating period during which as much of the moisture as possible is eliminated before the pressure is applied to form the assembly, without the hazard of precuring the adhesive.

To illustrate this process I will describe the production of corrugated Wood with paper laminated to either side so that the wood will not split while being further processed.

Using a inch sheet of hardwood averaging 8% moisture content and weighing approximately 150 pounds per 1,000 square feet, the sheet is arranged with the grain of the wood generally parallel to the axis of the corrugation to be formed. The sheet is then coated on both curfaces with an adhesive mix of ureaformaldehyde of 50% solids content and not over 10% by weight of the wet mix of hardener or catalyst such as ammonium chloride.

The adhesive is spread uniformly on the surface of the wood sheet at a rate, on each side of the sheet of not less than 8 nor more than 12 pounds of wet glue per 1,000 square feet of single surface area of the wood.

Sheets of high finish, low porosity, kraft paper of 60-70 pounds basic weight of a moisture content of 5 to 6 percent, by weight of the paper itself, are then applied, one sheet to either side of the glue coated surfaces'of the wood sheet. Next the assembly, in flat form, is. preheated for not over 30 seconds in a chamber of dry heated air at 25.0 F. to eliminate about percent of the moisture contents of the paper, thewet glue and the wood. Theassembled unit of three sheets is then immediately passed into a set of heated corrugating pressure dies at a temperature of 285 to 300 F. and pressure of 30-50 p. s. i. applied for a period of not less than 20 seconds. Pressure as long as 30 seconds tends to improve the set and hardness of the product but very little additional hard ness is achieved beyond a 40 second period-mot sutficient to be of practicable benefit in this particular product.

From the foregoing it will be appreciated that my method or process contemplates what I term broadly as dry bending of wood and that while I have described the technique that would be employed in making a specific novel corrugated wood product having laminations of other materials, the broad invention herein includes obvious modifications which may be resorted to in producing other useful products, which like that herein disclosed can be used in the manufacture of various articles, such as door panels, furniture elements and other items too numerous to mention.

What is claimed is:

1. A process of producing a. laminated corrugated sheet material having at least one lamination of wood, comprising the steps of arranging a sheet of wood With the grain of the wood generally parallel to the axis of the corrugation to be formed; coating the surfaces of said sheet with a wet adhesive mix of urea-formaldehyde of 50% solids content and an ammonium chloride hardener or catalyst of not over 10% by Weight of the wet mix; spreading said coating uniformly on the surfaces of the wood sheet, at a rate, on each side of the sheet of not less than 8 nor more than 12 pounds of wet glue per 1,000 square feet of single surface area of the wood; applying to each coated surface of the wood sheet, a sheet of paper of moisture content of 5 to 6 percent by weight of the paper; preheating the said assembly in fiat form for not over 30 seconds in a chamber of dry heated air'at 250 degrees Fahrenheit until about 90% of the moisture contents of the paper, wet coating, and wood is eliminated; and then immediately subjecting said assembly to heated corrugat ing die pressure at a temperature of 285 to 300 degrees Fahrenheit and pressure of 30 to 50 pounds per square inch for a period of not less than 20 seconds.

2. A process as claimed in claim 1 wherein the final heat and pressure step thereof polymerizes the lignin content of the wood.

References Cited in the file of this patent OTHER REFERENCES Serial No. 185,492, Frank (A. P. 0; published April. 

